Programming methods for multi-level memory devices

ABSTRACT

A method is provided for programming a memory cell. The memory cell is fabricated on a substrate and comprises a source region, a drain region, a floating gate, and a control gate. The memory cell has a threshold voltage selectively configurable into one of at least three programming states. The method includes generating a drain current between the drain region and the source region by applying a drain-to-source bias voltage between the drain region and the source region. The method further includes injecting hot electrons from the drain current to the floating gate by applying a gate voltage to the control gate. A selected threshold voltage for the memory cell corresponding to a selected one of the programming states is generated by applying a different selected gate voltage.

CLAIM OF PRIORITY

This application is a continuation application of, and claims priorityto, U.S. patent application Ser. No. 11/496,969, filed Aug. 1, 2006,which is a continuation application of, and claims priority to U.S.patent application Ser. No. 10/999,030, filed Nov. 29, 2004, (now U.S.Pat. No. 7,085,164, issued Aug. 1, 2006) which is a continuationapplication of, and claims priority to, U.S. patent application Ser. No.10/324,653, filed Dec. 18, 2002, (now U.S. Pat. No. 6,845,039, issuedJan. 18, 2005) which is incorporated in its entirety by reference hereinand which is a divisional application of, and claims priority to U.S.patent application Ser. No. 09/920,866, filed Aug. 2, 2001 (now U.S.Pat. No. 6,522,584, issued Feb. 18, 2003), which is incorporated in itsentirety by reference herein.

CROSS-REFERENCE TO RELATED CO-PENDING APPLICATION

This application is related to U.S. patent application Ser. No.10/998,697, filed Nov. 29, 2004 (now U.S. Pat. No. 7,035,145, issuedApr. 25, 2006), which is incorporated in its entirety by referenceherein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to electrically reprogrammablenonvolatile memory devices and methods of utilizing the same. Moreparticularly, the invention relates to processes and structures forprogramming erasable programmable read-only memories (EEPROMs).

2. Description of the Related Art

Memory devices such as erasable programmable read-only memories(EPROMs), electrically erasable programmable read-only memories(EEPROMs), or flash erasable programmable read-only memories (FEPROMs)are erasable and reusable memory cells which are often used in digitalcellular phones, digital cameras, LAN switches, cards for notebookcomputers, etc. A memory cell operates by storing electric charge(representing either a binary “1” or “0” state of one data bit) on anelectrically isolated floating gate, which is incorporated into atransistor. This stored charge affects the threshold voltage (V_(T)) ofthe transistor, thereby providing a way to read the memory element. Itis therefore crucial that the memory cell be able to maintain the storedcharge over time, so that charge leakage does not cause data errors byconverting the data bit from one binary state to another.

A memory cell typically consists of a transistor, a floating gate, and acontrol gate above the floating gate in a stacked gate structure. Thefloating gate, typically composed of polycrystalline silicon (i.e.,“polysilicon”), is electrically isolated from the underlyingsemiconductor substrate by a thin dielectric layer, which is typicallyformed of an insulating oxide, and more particularly, silicon oxide.This dielectric layer is often referred to as a “tunnel oxide” layer,and is typically approximately 100 Å thick. Properties of the tunneloxide layer must be strictly controlled to ensure the ability to readand write by transferring electrons across the tunnel oxide layer, whileavoiding data loss through charge trapping or leakage. The control gateis positioned above the floating gate, and is electrically isolated fromthe floating gate by a storage dielectric layer, such asoxide-nitride-oxide (ONO). Electrical access to the floating gate istherefore only through capacitors.

A programmed memory cell has its V_(T) increased by increasing theamount of negative charge stored on the floating gate, i.e., for givensource and drain voltages, the control gate voltage which allows acurrent to flow between the source and the drain of a programmed memorycell is higher than that of a non-programmed memory cell. Therefore, thestate of a memory cell is read by applying a control gate voltage belowthe predetermined level corresponding to the programmed state, butsufficiently high to allow a current between the source and the drain ina non-programmed memory cell. If a current is detected, then the memorycell is read to be not programmed.

One method to erase a memory cell (i.e., return the cell to itsnon-programmed state) is by exposing the floating gate to ultravioletlight, which excites the stored electrons out of the floating gate. Theerasure of an EEPROM or FEPROM cell can also be accomplished viaFowler-Nordheim tunneling of charge from the floating gate, across thetunnel oxide, to the substrate, thereby reducing the stored charge inthe floating gate. Under this mechanism for discharging the floatinggate, for example, a large negative voltage (e.g., −10 V) is applied tothe control gate, and a positive voltage (e.g., 5-6 V) is applied to thesource while the drain is left floating. Electrons then tunnel from thefloating gate through the tunnel oxide, and are accelerated into thesource.

In an attempt to increase the storage density of an array of memorycells, efforts have been made to utilize multilevel memory cells, whichare capable of representing more than two states by specifying more thanone predetermined V_(T) level. In such multilevel memory cells, eachrange of levels defined by the predetermined V_(T) levels corresponds toa separate state. Therefore, to reliably distinguish between the variousstates, the multilevel memory cells must be programmed with narrow V_(T)distributions within the ranges defined by the predetermined V_(T)levels. Traditionally, these narrow V_(T) distributions have beenachieved using short programming pulses interleaved with verificationread pulses in order to closely monitor the programmed level of a givencell. Examples of such multilevel memory cell programming are disclosedby Kucera, et al., U.S. Pat. No. 6,091,631; Fazio, et al., U.S. Pat. No.5,892,710; and Harari, U.S. Pat. No. 5,293,560.

Such use of verification steps has two potential drawbacks. First, thecircuitry needed to confirm that a particular cell has been properlyprogrammed takes up valuable space on the semiconductor die. Second, thefrequent verification steps take a substantial amount of time, therebyprolonging the programming process.

SUMMARY OF THE INVENTION

By eliminating the verification steps, the present invention achievesfaster multi-level programming of flash memory devices. In accordancewith one aspect of the present invention, a method is provided forprogramming a memory cell of an electrically erasable programmable readonly memory. The memory cell is fabricated on a substrate and comprisesa source region, a drain region, a floating gate, and a control gate.The memory cell has a threshold voltage selectively configurable intoone of at least three programming states. The method comprisesgenerating a drain current between the drain region and the sourceregion by applying a drain-to-source bias voltage between the drainregion and the source region. The method further comprises injecting hotelectrons from the drain current to the floating gate by applying a gatevoltage to the control gate. A selected threshold voltage for the memorycell corresponding to a selected one of the programming states isgenerated by applying a selected constant drain-to-source bias voltage.

In accordance with another aspect of the present invention, a method isprovided for programming a memory cell of an electrically erasableprogrammable read only memory. The memory cell is fabricated on asubstrate and comprises a source region, a drain region, a floatinggate, and a control gate. The memory cell has a threshold voltageselectively configurable into one of at least three programming states.The method comprises generating a drain current between the drain regionand the source region by applying a drain-to-source bias voltage betweenthe drain region and the source region. The drain-to-source bias voltagecomprises at least one voltage pulse. The method further comprisesinjecting hot electrons from the drain current to the floating gate byapplying a gate voltage to the control gate. A selected thresholdvoltage for the memory cell corresponding to a selected one of theprogramming states is generated by applying a selected drain-to-sourcebias voltage.

In accordance with yet another aspect of the present invention, a methodis provided for programming a memory cell of an electrically erasableprogrammable read only memory. The memory cell is fabricated on asubstrate and comprises a source region, a drain region, a floatinggate, and a control gate. The memory cell has a threshold voltageselectively configurable into one of at least three programming states.The method comprises generating a drain current between the drain regionand the source region by applying a drain-to-source bias voltage betweenthe drain region and the source region. The drain-to-source bias voltagecomprises at least one voltage pulse. The method further comprisesinjecting hot electrons from the drain current to the floating gate byapplying a gate voltage to the control gate. A selected thresholdvoltage for the memory cell corresponding to a selected one of theprogramming states is generated by applying a selected gate voltage.

In accordance with yet another aspect of the present invention, a methodis provided for programming a memory cell of an electrically erasableprogrammable read only memory. The memory cell is fabricated on asubstrate and comprises a source region, a drain region, a floatinggate, and a control gate. The memory cell has a threshold voltageselectively configurable into one of at least three programming states.The method comprises generating a drain current between the drain regionand the source region by applying a drain-to-source bias voltage betweenthe drain region and the source region. The method further comprisesinjecting hot electrons from the drain current to the floating gate byapplying a gate voltage to the control gate. The gate voltage comprisesat least one voltage pulse. A selected threshold voltage for the memorycell corresponding to a selected one of the programming states isgenerated by applying a selected gate voltage.

In accordance with yet another aspect of the present invention, a methodis provided for programming a memory cell of an electrically erasableprogrammable read only memory. The memory cell is fabricated on asubstrate and comprises a source region, a drain region, a floatinggate, and a control gate. The memory cell has a threshold voltageselectively configurable into one of at least three programming states.The method comprises generating a drain current between the drain regionand the source region by applying a drain-to-source bias voltage betweenthe drain region and the source region. The method further comprisesinjecting hot electrons from the drain current to the floating gate byapplying a gate voltage to the control gate. The gate voltage is rampedfrom an initial magnitude to a final magnitude greater than the initialmagnitude, and the gate voltage is ramped with a ramping rate. Aselected threshold voltage for the memory cell corresponding to aselected one of the programming states is generated by applying a gatevoltage with a selected final magnitude.

In accordance with yet another aspect of the present invention, a methodis provided for programming a memory cell of an electrically erasableprogrammable read only memory. The method comprises selectivelyconfiguring the memory cell into one of at least three programmingstates without a verification step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a semiconductor substrate with a memorycell compatible with the present invention.

FIG. 2 schematically illustrates channel hot-electron (CHE) injection asa method of charge transfer to the floating gate.

FIG. 3 schematically illustrates an array of NOR-type flash memories.

FIG. 4 schematically illustrates Fowler-Nordheim (FN) tunneling as amethod of charge transfer to the floating gate.

FIG. 5 schematically illustrates an array of NAND-type flash memories.

FIG. 6 schematically illustrates the variation of floating gate chargeas a function of time upon applying voltages corresponding to CHEinjection to the memory cell.

FIG. 7A schematically illustrates the multilevel states achieved byusing different drain voltages during the CHE injection operation.

FIG. 7B schematically illustrates the threshold voltage achieved as afunction of time by applying different drain voltages during the CHEinjection operation.

FIG. 8 schematically illustrates the multilevel states achieved by usingdifferent gate voltages during the CHE injection operation.

FIG. 9 schematically illustrates the multilevel states achieved by usingdifferent pulse widths on the voltages during the CHE injectionoperation.

FIG. 10 schematically illustrates the variation of the drain current asa function of time upon applying a ramped gate voltage as compared to aconstant gate voltage.

FIG. 11 schematically illustrates the multilevel states achieved byusing different ramping rates on the gate voltage.

FIG. 12 schematically illustrates the drain region of a memory cellconnected to a transistor.

FIG. 13 schematically illustrates the drain region of a memory cellconnected to a plurality of pairs of transistors and resistors, thepairs connected in parallel.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 schematically illustrates a semiconductor substrate 10 with amemory cell 20 compatible with the present invention. The memory cell 20includes a transistor 30, a floating gate 40, and a control gate 50above the floating gate 40 in a stacked gate structure. The transistor30 comprises a source region 32, a drain region 34, and a channel 36between the source region 32 and the drain region 34. The floating gate40, typically composed of polycrystalline silicon (i.e., “polysilicon”),is electrically isolated from the underlying semiconductor substrate 10by a tunnel dielectric layer 60, which is a thin dielectric layer,typically formed of an insulating oxide, and more particularly, siliconoxide, and is typically approximately 100 Å thick. The control gate 50is positioned above the floating gate 40, and is electrically isolatedfrom the floating gate 40 by a storage dielectric layer 70, such asoxide-nitride-oxide (ONO). Electrical access to the floating gate 40 istherefore only through capacitors. It should be noted that the materialsmentioned herein are merely exemplary.

An array of memory cells can be programmed by transferring charge fromthe semiconductor substrate to the floating gates of selected memorycells. One method of achieving this charge transfer is via channelhot-electron (CHE) injection, which is a three-terminal process,schematically illustrated by FIG. 2. CHE injection utilizes a highpositive voltage (e.g., approximately 10 V) applied to the control gate50, grounding the source region 32 of the transistor 30, and applying ahigh positive voltage (e.g., approximately 5 V) to the drain region 34of the transistor 30, thereby creating a high drain-to-source biasvoltage (e.g., approximately 5 V). An inversion region is created in thechannel 36 between the source 32 and drain 34 regions by the gatevoltage, and electrons 80 are accelerated from the source region 32 tothe drain region 34 by the drain-to-source bias voltage, therebycreating a drain current between the source region 32 and drain region34. Some fraction 82 of the electrons 80 will have sufficient energy tosurmount the barrier height of the tunnel dielectric 60 and reach thefloating gate 40, thereby charging the floating gate 40. By collectingand storing a sufficient amount of charge on the floating gate 40, theV_(T) of the transistor 30 is increased to be above a predeterminedlevel corresponding to a programmed binary state (e.g., “0”). A memorycell 20 with no charge on its floating gate 40, and with acorrespondingly lower V_(T), is in a non-programmed state (e.g., “1”).

CHE programming is typically used to program NOR-type flash memories, asschematically illustrated in FIG. 3. In NOR-type flash memories, thememory cells 20 are connected in parallel with the drain region 34 ofeach memory cell 20 connected to a respective bit-line 90 and with thesource region 32 of each memory cell connected to a common source line(not shown). A select line 92 connects the control gates 50 of a columnof memory cells 20, one on each bit-line 90, allowing several memorycells 20, such as a byte or a word, to be accessed in parallel. WhileNOR-type flash memories provide fast random access, its parallelstructure reduces its memory density.

Another method of storing charge on the floating gate 40 is viaFowler-Nordheim (FN) tunneling, schematically illustrated in FIG. 4,which is a two-terminal process and does not utilize a drain current 80between the source 32 and drain 34 regions of the transistor 20. Byapplying a high positive voltage (e.g., approximately 10 V) to thecontrol gate 50, a high negative voltage (e.g., approximately −10 V) tothe p-well substrate 10 containing the flash memory cell 20, andfloating the source 32 and drain 34 regions, an electric field iscreated which is sufficient for electrons 84 to tunnel through thetunnel dielectric 60 from the p-well substrate 10 and to enter thefloating gate 40, thereby programming the memory cell 20. FN tunnelingcan occur in any two terminal device (such as a capacitor) and does notrequire a drain-to-source bias voltage. Compared to CHE programming, FNprogramming requires higher voltages and has slower programming speeds(typically requiring greater than 1 ms to program as compared to a fewμs for CHE programming). However, the lower current densities of FNprogramming make it easier to use as a method of programming many flashmemory cells at the same time (i.e., in parallel).

FN tunneling is typically used to program NAND-type flash memories, asschematically illustrated in FIG. 5. In NAND-type flash memories, thememory cells 20 are connected serially to form strings 96 of memorycells 20, with each string 96 having a string select gate 98 at one endwhich connects the string 96 to a bit-line 100 and a ground select gateat the opposite end of the string which connects the string to ground(not shown). A select line 102 connects the control gates 50 of a columnof memory cells 20, one on each string 96. Because no outside contact isneeded within a string 96 of memory cells 20, NAND-type flash memoriesprovide increased memory density. However, because selected memory cells20 must be accessed through the other unselected memory cells 20 of thestring 96, the reading speed is limited.

As described above, the traditional programming method for multilevelmemory cells has been to program the memory cell using short programmingpulses of the control gate voltage and drain voltage. To achieve thenarrow V_(T) distributions, the short programming pulses are typicallyinterleaved with verification read pulses in order to closely monitorthe programmed level of a given cell. The short programming pulses andfrequent verification steps make the program operation very timeconsuming.

In addition, use of CHE injection to program flash EPROM devices has thedisadvantage of requiring rather high drain currents in order togenerate the charge which is stored on the floating gate 40. Because thetotal current available to program the memory cells is limited at anyone time, the high drain currents constrain the number of memory cells20 which can be programmed in parallel to typically a few hundred cellsmaximum. One way to reduce the peak programming current, as disclosed byKeeney, et al. in U.S. Pat. Nos. 5,553,020 and 5,487,033, is to step thegate voltage of the memory cell 20 as it is being programmed, inincrements from an initial minimum value to a maximum value based uponthe number of levels to program in the memory cell 20. Each step of thegate voltage is accompanied by a corresponding pulse of the drainvoltage and a verifying pulse until the desired threshold voltage isreached on the floating gate. In this way, the drain current can bereduced; however, there is a corresponding increase in the time requiredto charge the floating gate 40 to a given level.

Similarly, while use of FN tunneling to program flash EPROM devices doesnot utilize drain current, stepping up the gate voltage of the memorycell 20 as it is being programmed reduces the tunneling current andsignificantly increases the time required to program the memory cell 20.The programming current can be further reduced by applying a reverseback bias between the substrate 10 and the control gate 50 to increasethe gate current efficiency, as is described by Hu, et al., in“Substrate-Current-Induced Hot Electron (SCIHE) Injection: A NewConvergence Scheme For Flash Memory,” IEDM Tech. Dig., pp. 283-286,1995, which is incorporated by reference herein.

The charge Q applied to the floating gate 40 varies with time during thecharging from an initial value of Q₀ (equal to zero prior to applyingthe voltages corresponding to CHE injection at t₀) to a saturation valueQ₁ at a later time t_(∞). FIG. 6 schematically illustrates the variationof floating gate charge Q as a function of time upon applying voltagescorresponding to CHE injection to the memory cell 20. As more charge isbuilt up in the floating gate 40, the electric field which attracts thehot electrons in the channel 36 toward the floating gate 40 is lessened.Once the floating gate 40 has the saturation charge Q₁, the hotelectrons in the channel 36 are shielded from the gate voltage V_(G) andno more hot electrons reach the floating gate 40. The amount ofsaturation charge Q₁ is dependent on the gate voltage V_(G), as isapparent from FIG. 6, which shows three different saturation chargelevels for three different gate voltages. The time to reach thesaturation charge Q₁ does not exhibit a large dependency on the gatevoltage V_(G), but it is typically between approximately 100 ns and 10μs. The threshold voltage V_(T) is dependent on the amount of chargeapplied to the floating gate 40.

In certain embodiments of the present invention, the memory cell 20 isprogrammed using CHE injection by applying a constant drain-to-sourcebias voltage between the drain region 34 and source region 32 and a gatevoltage to the control gate 50 without verification read pulses.Selected threshold voltages for the memory cell 20 corresponding to aselected one of the multilevel programming states are generated byapplying different selected drain-to-source bias voltages during the CHEinjection operation. In one such embodiment, schematically illustratedin FIG. 7A, each cell has two bits, and hence four states: (00), (01),(10), and (11). In certain embodiments, one of the four states (e.g.,(00)) corresponds to the cell after erase. The magnitude of the drainvoltage V_(D) is selected to provide a selected threshold voltagecorresponding to one of the three other possible states: (01), (10), and(11), with the gate voltage V_(G) and the source voltage V_(S) set atconstant values (e.g., V_(G) is set to approximately +10 V, V_(S) is setto approximately 0 V). The amount of charge injected onto the floatinggate 40 corresponds to the potential difference between the sourceregion 32 and the drain region 34, i.e., the drain-to-source biasvoltage. For example, referring to FIG. 7A, to apply a charge to thefloating gate 40 of a memory cell 20 so that the threshold voltagecorresponds to a (01) state, the drain voltage V_(D) is set to a voltagesubstantially equal to V_(D1). Similarly, the (10) state is achieved byapplying a drain voltage that is substantially equal to V_(D2), and the(11) state is achieved by applying a drain voltage that is substantiallyequal to V_(D3).

Referring to FIG. 7B, various values of the drain voltage V_(D) (e.g.,V_(D1)=4 V; V_(D2)=5 V; V_(D3)=6V) produce different threshold voltagesV_(t) after a fixed program time. By halting the programming of a cellafter a fixed time, embodiments of the present invention achieveselected multilevel programming states while avoiding the verificationsteps of the prior art.

In other embodiments of the present invention, selected thresholdvoltages corresponding to selected multilevel programming states aregenerated by applying different gate voltages during the CHE injectionoperation. As schematically illustrated in FIG. 8, the memory cell 20 isprogrammed using CHE injection by setting the drain voltage and thesource voltage to constant values (e.g., V_(D) of approximately +5 V andV_(S) of approximately 0 V). The magnitude of the gate voltage V_(G) isselected to provide a selected threshold voltage corresponding to one ofthe four possible multilevel states. The amount of charge injected ontothe floating gate 40 corresponds to the gate voltage. For example,referring to FIG. 8, to apply a charge to the floating gate 40 of amemory cell 20 so that the threshold voltage corresponds to a (01)state, the gate voltage V_(G) is set to a voltage substantially equal toV_(G1) (e.g., V_(G1)=6 V). Similarly, the (10) state is achieved byapplying a gate voltage substantially equal to V_(G2) (e.g., V_(G2)=8V), and the (11) state is achieved by applying a gate voltagesubstantially equal to V_(G3) (e.g., V_(G3)=10 V).

In still other embodiments of the present invention, the time profile ofthe applied voltages, as schematically illustrated in FIG. 9, isutilized to set the threshold voltage to one of the four possiblestates. A selected threshold voltage corresponding to a selectedmultilevel programming state is generated by applying voltagescomprising at least one voltage pulse, with a selected pulse periodduring the CHE injection operation. For example, referring to FIG. 9, toapply a charge to the floating gate 40 of a memory cell 20 so that thethreshold voltage corresponds to a (01) state, a single gate voltagepulse V_(G) with a magnitude of approximately 10 V and a period ofapproximately (t₁−t₀) is applied to the control gate 50 while the sourcevoltage and drain voltage are set at constant values (e.g., V_(S) ofapproximately 0 V, V_(D) of approximately 5 V). Similarly, the (10)state is achieved by applying a voltage pulse with a periodapproximately equal to (t₂−t₀), and the (11) state is achieved byapplying a voltage pulse with a period approximately equal to (t₃−t₀).In certain embodiments, the total time in which the appropriate voltagesfor CHE injection are applied to the memory cell 20 are distributedamong a plurality of pulses (i.e., the sum of the pulse periods of theplurality of pulses has the required time duration to achieve thedesired state of the memory cell 20). The voltage pulse of the gatevoltage V_(G) preferably has a period of between approximately 1 ns andapproximately 10 μs. For example, V_(G) can be applied for 2 ns toprogram the (01) state, 0.1 μs to program the (10) state, and 2 μs toprogram the (11) state.

Alternatively, in other embodiments, the drain voltage V_(D) can bepulsed and the gate voltage V_(G) selected to generate a selectedthreshold voltage corresponding to a selected multilevel programmingstate. For example, V_(D) can be applied for 2 ns to program the (01)state, 0.1 μs to program the (10) state, and 2 μs to program the (11)state. The voltage pulse of the drain voltage V_(D) preferably has aperiod of between approximately 1 ns and approximately 10 μs.Alternatively, in still other embodiments, the drain voltage V_(D) canbe pulsed, the source voltage V_(S) can be pulsed, or any combination ofthe gate voltage V_(G), drain voltage V_(D), and source voltage V_(S)can be pulsed to set the threshold voltage to one of the four possiblestates.

In still other embodiments of the present invention, selected thresholdvoltages corresponding to selected multilevel programming states aregenerated by ramping the voltages applied to the memory cell 20 duringthe CHE injection operation without applying verification pulses. Asdescribed above, as charge accumulates in the floating gate 40, the hotelectrons in the channel 36 are increasingly shielded from the gatevoltage V_(G), thereby reducing the rate of charge injection andeventually reaching a saturation level corresponding to a saturationthreshold voltage. When applying a constant gate voltage V_(G), asschematically illustrated in FIG. 10, there will be a high drain currentat the beginning of the programming, due to the large potentialdifference between the floating gate 40 and the channel 36. By rampingthe gate voltage V_(G) during the CHE injection from an initialmagnitude to a final magnitude greater than the initial magnitude, thedrain current will be constant over an extended period of time, at alower level, as schematically illustrated in FIG. 10. The saturationthreshold will be determined by both the drain voltage, and the finalgate voltage at the end of the ramp.

In certain embodiments, a ramped gate voltage V_(G) is used withdifferent values of the drain voltage V_(D), the values of the applieddrain voltage V_(D) selected to generate selected threshold voltages forthe memory cell 20 corresponding to selected multilevel programmingstates. Such embodiments are similar to those discussed in relation toFIG. 7, but with lessened drain currents due to the ramping of the gatevoltage V_(G), so that a large number of cells can be programmed inparallel. In certain other embodiments, a ramped gate voltage V_(G) isused with different pulse periods of the drain voltage V_(D), the pulseperiods selected to generate selected threshold voltages correspondingto selected multilevel programming states of the memory cell 20. Suchembodiments are similar to those discussed in relation to FIG. 9, butwith lessened drain currents due to the ramping of the gate voltageV_(G), so that a large number of cells can be programmed in parallel. Instill other embodiments, the value of the final gate voltage V_(G)during the ramping is selected to generate selected threshold voltagescorresponding to selected multilevel programming states of the memorycell 20. Such embodiments are similar to those discussed in relation toFIG. 8, but with lessened drain currents due to the ramping of the gatevoltage V_(G), and again a large number of cells can be programmed atthe same time. For example, 7 V, 9 V, and 11 V can be used as the finalgate voltage for programmed states (01), (10), and (11), respectively.

As schematically illustrated in FIG. 11, by selecting the rate oframping of the gate voltage V_(G), selected threshold voltagescorresponding to selected multilevel programming states of the memorycell 20 can be generated. The amount of charge on the floating gate 40after CHE injection for a time (t₁−t₀) is dependent on the ramping rateof the gate voltage V_(G). Slower ramping rates correspond to slowercharge injection on the floating gate 40, and less charge on thefloating gate 40 after a known time (t₁−t₀). Therefore, the ramping rateof V_(G) can be selected to yield a particular amount of charge on thefloating gate 40 after a known charging time, thereby setting thethreshold voltage to one of the four possible states. For example,within 1 ms, the gate voltage can be ramped from 0 V to 6 V, 8 V, and 10V for programmed states (01), (10), and (11), respectively. In certainembodiments, the injection of charge is performed for the known time(t₁−t₀) by pulsing the voltages applied to the memory cell 20 (e.g.,pulsing the drain voltage V_(D), the source voltage V_(S), the gatevoltage V_(G), or any combination of the gate voltage V_(G), drainvoltage V_(D), and source voltage V_(S)).

In certain other embodiments, where the drain voltage does not have aconstant magnitude, the drain region 34 can be connected to a constantcurrent source. In this way, the drain current is maintained to have asubstantially constant magnitude throughout the charging operation. Theamount of charge injected onto the floating gate 40 varies with theamount of drain current, so multilevel states are generated by usingdifferent drain currents during the CHE injection operation. Such aconstant drain current can be utilized with any of the above-describedembodiments.

In still other embodiments, a reverse back bias can be applied betweenthe substrate 10 and control gate 50 to increase the fraction of hotelectrons which are injected from the channel 36 to the floating gate40. When combined with the above-described embodiments, the reverse backbias improves the programming efficiency and yields faster convergenceto the desired saturation threshold voltage. Values of the back biascompatible with embodiments of the present invention range fromapproximately 0 V to approximately −4V.

Different voltages on the drain region 34 can be achieved by connectinga transistor 110 in series between the drain region 34 of the memorycell 20 and a drain voltage generator (not shown). For example, asschematically illustrated in FIG. 12, a transistor 110 is connected on abit line in series to the drain region 34, the transistor 110 having atransistor control gate 112. Multilevel states are achieved by usingdifferent control voltages on the transistor control gate 112 during theprogramming operation to adjust a voltage applied to the drain region 34of the memory cell 20. Such a transistor 110 can be utilized with any ofthe above-described embodiments.

In alternative embodiments, as schematically illustrated in FIG. 13, aplurality of pairs of transistors 120 and resistors 124, in which eachtransistor 120 has a transistor control gate 122 and is in series with acorresponding resistor 124, are connected in parallel between the drainregion 34 of the memory cell 20 and a drain voltage generator (notshown). The resistors 124 each have a different resistance, andmultilevel states are achieved by selectively applying a control voltageto at least one transistor 120, thereby applying different voltages tothe drain region 34 during the programming operation. Such a pluralityof pairs of transistors 120 and resistors 122 can be utilized with anyof the above-described embodiments.

In the embodiments described above, the memory cells are initiallydischarged or erased, and the appropriate amount of charge is applied tothe floating gate 40 to correspond to one of the multilevel programmingstates. In still other embodiments compatible with the presentinvention, the memory cells are initially charged to a selected value,and then discharged by a selected amount, resulting in the appropriateamount of charge on the floating gate 40 to correspond to one of themultilevel programming states.

Although described above in connection with particular embodiments ofthe present invention, it should be understood the descriptions of theembodiments are illustrative of the invention and are not intended to belimiting. Various modifications and applications may occur to thoseskilled in the art without departing from the true spirit and scope ofthe invention as defined in the appended claims.

1. A method for selecting a threshold voltage corresponding to one of atleast three programming states of a memory cell, the memory cellfabricated on a substrate and comprising a source region, a drainregion, a floating gate, and a control gate, the method comprising:generating a drain current between the drain region and the sourceregion by applying a drain-to-source bias voltage between the drainregion and the source region, the drain-to-source bias voltagecomprising at least one voltage pulse; and generating a selectedthreshold voltage for the memory cell corresponding to a selected one ofthe programming states by injecting hot electrons from the drain currentto the floating gate by applying a gate voltage to the control gate,whereby the selected threshold voltage for each programming state isgenerated by applying a different selected gate voltage.
 2. The methodof claim 1, wherein the drain-to-source bias voltage maintains a draincurrent having a substantially constant magnitude.
 3. (canceled) 4.(canceled)
 5. The method of claim 1, wherein a reverse back bias isapplied between the substrate and the control gate.
 6. The method ofclaim 1, wherein the selected gate voltage is applied for a selectedperiod of time.
 7. (canceled)
 8. The method of claim 1, wherein theselected gate voltage is ramped from an initial magnitude to a finalmagnitude greater than the initial magnitude, the selected gate voltagebeing ramped with a ramping rate.
 9. The method of claim 8, wherein theramping rate is between approximately 5.5 V/ms and approximately 10.5V/ms.
 10. (canceled)
 11. The method of claim 1, wherein the voltagepulse of the drain-to-source bias voltage is applied while the selectedgate voltage is applied.
 12. The method of claim 1, wherein thedrain-to-source bias voltage is approximately 5 volts.
 13. (canceled)14. A method for programming a memory cell, the memory cell fabricatedon a substrate and comprising a source region, a drain region, afloating gate, and a control gate, the memory cell having a thresholdvoltage selectively configurable into one of at least three programmingstates, the method comprising: generating a drain current between thedrain region and the source region by applying a drain-to-source biasvoltage between the drain region and the source region, thedrain-to-source bias voltage comprising at least one voltage pulse; andgenerating a selected threshold voltage for the memory cellcorresponding to a selected one of the programming states by injectinghot electrons from the drain current to the floating gate by applying agate voltage to the control gate, whereby the selected threshold voltagefor each programming state is generated by applying a different selectedgate voltage, wherein the selected gate voltage is ramped from aninitial magnitude to a final magnitude greater than the initialmagnitude, the selected gate voltage being ramped with a ramping rate.15. The method of claim 14, wherein the drain-to-source bias voltagemaintains a drain current having a substantially constant magnitude. 16.The method of claim 14, wherein a transistor comprising a control gateis connected in series between the drain region of the memory cell and adrain voltage generator, whereby a control voltage applied to thecontrol gate of the transistor adjusts a voltage applied to the drainregion of the memory cell.
 17. The method of claim 14, wherein aplurality of circuit segments are connected in parallel between thedrain region of the memory cell and a drain voltage generator, eachcircuit segment comprising a resistor connected in series with atransistor having a control gate, whereby selectively applying a controlvoltage to the control gate of at least one transistor adjusts a voltageapplied to the drain region of the memory cell.
 18. The method of claim14, wherein a reverse back bias is applied between the substrate and thecontrol gate.
 19. The method of claim 14, wherein the ramping rate isbetween approximately 5.5 V/ms and approximately 10.5 V/ms.
 20. Themethod of claim 14, wherein the final magnitude is between approximately5.5 V and approximately 10.5 V.
 21. A method for programming a memorycell, the memory cell fabricated on a substrate and comprising a sourceregion, a drain region, a floating gate, and a control gate, the memorycell having a threshold voltage selectively configurable into one of atleast three programming states, the method comprising: generating adrain current between the drain region and the source region by applyinga drain-to-source bias voltage between the drain region and the sourceregion, the drain-to-source bias voltage comprising at least one voltagepulse; and injecting hot electrons from the drain current to thefloating gate by applying a gate voltage to the control gate, whereby aselected threshold voltage for the memory cell corresponding to aselected one of the programming states is generated by applying adifferent selected gate voltage.
 22. The method of claim 21, wherein theselected gate voltage is ramped from an initial magnitude to a finalmagnitude greater than the initial magnitude, the selected gate voltagebeing ramped with a ramping rate.
 23. The method of claim 22, whereinthe ramping rate is between approximately 5.5 V/ms and approximately10.5 V/ms.
 24. The method of claim 22, wherein the final magnitude isbetween approximately 5.5 V and approximately 10.5 V.
 25. The method ofclaim 21, wherein the drain-to-source bias voltage maintains a draincurrent having a substantially constant magnitude.
 26. (canceled) 27.(canceled)
 28. (canceled)